It has been ruled that from the year of 2015, a new fuel efficiency consumption regulation would be imposed on domestic automobiles, and an improvement in the fuel saving properties of an automobile has been demanded increasingly year by year. Lubricating oils used for the transmissions and internal combustion engines of automobiles have been demanded to contribute to energy saving, and lowering of the viscosity of a lubricating oil is an example of the contribution. Lowering of the viscosity of a lubricating oil to be used for example in a transmission can reduce the stir and frictional resistances and thus enhance the power transmission efficiency, resulting in an improvement in the fuel efficiency of an automobile. However, when the viscosity of a lubricating oil is lowered, the oil film thickness is decreased, resulting in degradations in anti-wear properties and anti-seizure properties and accordingly some modifications to the base oil and additives to be used is required (see Patent Literatures 1 and 2).
Whilst, a lubricating oil, in particular a transmissions oil is required to have long service life characteristics that can retain main properties such as anti-wear properties, anti-seizure properties and the like for a long period of time. However, sufficient care in respect of anti-wear properties and anti-seizure properties considering the long service life characteristics thereof after degradation of an oil composition has not been given to the transmission lubricating oil compositions disclosed in the above patent literatures.
With regard to the degradation, it has been found that other than degradation of a lubricating oil composition due to oxidation after the use thereof for a long period of time, compounds eluted from sealing materials, i.e., rubber materials used in a transmission adversely affect the anti-wear properties of the lubricating oil composition. It has become apparent that this also adversely affects the friction characteristics, particularly the retainability of anti-shudder properties particularly needed as an automatic transmission oil.